The armed units, from the Guardia Civil, a force under the command of the Spanish interior and defence ministries, arrived at about 8AM on Wednesday (20 September) at the semi-autonomous region's economy, foreign affairs, labour, and social affairs ministry buildings in Barcelona, Spanish media said.

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Oriol Junqueras, the Catalan vice president, said on Catalunya Radio that they were hunting for electoral posters, funds, and other materials designated for the vote on 1 October.

The gendarmes also detained Junqueras' right hand man, Josep Maria Jove, as well as two officials from the Catalan economy and budget departments, the media reports said.

Jordi Sanchez, the president of the Catalan National Assembly, called for "peaceful resistance" against the operation on Twitter.

The raids followed a Guardia Civil swoop on a postal firm, Unipost, in the Catalan town of Terrassa on Tuesday in which they seized tens of thousands of census cards to be used in the vote.

The Unipost operation saw about 200 protesters rush to the scene, with isolated scuffles and with flowers laid on anti-riot vans.

Tensions are likely to rise again on Wednesday when the first of 740 Catalan mayors, Marc Solsona from the town of Mollerussa, accused of abuse of office by Spanish prosecutors, goes on trial.

For his part, the Spanish foreign minister, Alfonso Dastis, accused the pro-independence side of "Nazi" tactics in a TV interview in the US the night before.

He said Franco, Spain's former fascist leader, had organised two referendums in his time.

"Referendums are a weapon of choice of dictators," Dastis, who was in New York for the UN general assembly, said according to the Bloomberg news agency.

He said the pro-independence side was trying to intimidate anti-independence figures.

"These people actually are taking some Nazi attitudes because they are putting up posters with the faces of mayors who are resisting their call to participate in this charade [the referendum]," he said.

"That's something you don't see in a democracy," Dastis said.

The Spanish authorities, on Monday, also seized control of Catalonia's finances in what Junqueras called "total irresponsibility" that would lead the region "to an administrative collapse".

"This means that from now on [Catalan leaders] no longer have their money," a Spanish budget ministry spokeswoman said.

Francisco de la Torre, an MP from Spain's centrist Ciudadanos party, said: "Does this mean they [Catalans] have less autonomy? Of course. But the seriousness of the measure goes hand in hand with the seriousness of events".

The Spanish constitutional court has said the referendum would be illegal.

The European Commission has endorsed this view, but some individual EU states have taken a more nuanced position.

"The will of the people is always what matters, that is the position [of Hungary]," Hungary's government spokesman, Zoltan Kovacs, told reporters in Brussels on Monday.

He said the question of legality was "an internal issue for the Spanish and Catalan people," but added that democracy meant "to respect the will of the people".

Russian media and pro-Russian agitators have also backed the secessionist movement, amid concern in Madrid that it was losing international sympathy.

Junior coalition partner Most-Hid wants Slovaks to vote for a new parliament, after the killing of a journalist. "If talks about early elections fail, Most-Hid will exit the ruling coalition," its leader Bela Bugar said.